A surveillance radar system comprises a Primary Surveillance Radar (PSR) and an Identification Friend or Foe/Secondary Surveillance Radar (IFF/SSR). In prior art solutions, the IFF/SSR-antenna system typically consists of one or more separate antennas.
In a radar surveillance system, the PSR antenna will have a very narrow, main beam and extremely low side lobes. The IFF/SSR antenna has an operating frequency which normally is a few times lower than the operating frequency of the PSR. It is normally desired to have as large aperture as possible, measured in wavelengths, for both functions. One standard solution is to have two separate antenna apertures, which means an overall antenna system size, being the sum of the two antenna apertures. It would be desirable to use an increased aperture for the IFF/SSR-antenna without substantially increasing the overall antenna system size for a combined PSR and IFF/SSR antenna structure and without substantially degrading the PSR antenna performance. The arrays of the PSR and the IFF/SSR antennas may be electronically scanned which means that the direction of a main lobe can be electronically controlled. The PSR typically operates in a frequency band around one to several GHz.
U.S. Pat. No. 6,121,931 discloses a solution with a dual frequency array antenna having an essentially planar structure with electronic beam steering capability in both a low and a high frequency band independently of each other. The antenna is arranged in a layered formation, with a top planar array antenna unit operating in a low frequency band and a bottom planar array antenna unit operating in the high frequency band. The top planar array antenna is transparent to frequencies in the high frequency band. A drawback with this solution is that a rather complicated frequency selective surface for the radiating elements and ground plane of the top planar array antenna is required. A further drawback is that each antenna element in the top planar array antenna requires an individual feed, resulting in a complicated feeding network interfering with the bottom planar array antenna. The solution also has the limitation of using only patch elements in both bottom and top planar array antenna. The problem of achieving isolation between the two array antennas is solved by using frequency selective surfaces for the top planar array antenna. In order for such frequency selective surfaces to work as intended, they normally need to be very large, ideally infinite. In practice, the limited size will cause edge effects that will degrade the performance. This is a fairly complicated solution resulting in disturbances between the top and bottom planar array antennas degrading the high frequency performance.
FR 2734411, considered as closest prior art shows a solution where dipoles are interlaced with slots. The invention however seems to solve the problem to work with two different polarizations and not with two different frequency bands. The slots and dipoles are located in the same plane which creates a risk for interference between the two types of antenna elements. The feeding of the dipoles is complicated and/or includes parts of the feeding structure being parallel or almost parallel to the polarization of the slots. This feeding structure also increases the risk of increased interference between the different types of antenna elements. Furthermore, the substrate, used as a carrier for the microstrip transmission lines, will add losses to the slot antenna since it is located very close to the slot apertures.
There is thus a need to achieve an increased aperture for a low frequency antenna, as the IFF/SSR-antenna, without substantially degrading the PSR antenna performance and without substantially increasing the overall antenna system size for a combined high frequency, as the PSR antenna, and low frequency antenna structure while at the same time have an improved feeding of the antenna functions, and improved isolation between the antenna functions.